Stabilizing of cellulose esters



Patented Aug. 9, 1938 UNlTED STATES PATENT OFFICE pany, Jersey City, N. J

Jersey a corporationof New No Drawing. Application May 7, 1936, Serial No. 78,446'

3 Claims.

The present invention relates to the stabilizing of an organic acid ester of cellulose by washing the ester in a very dilute solution of a salt of magnesium or an alkali metal.

In the preparation of organic acid esters of cellulose, sulfuric acid is usually employed as the catalyst. The resulting esters usually contain a small sulfur residue which affects the stability of the ester. Various means have been suggested for stabilizing cellulose esters, such as boiling in distilled water, which involve the removal of this sulfur content.

I have found that the eifect of the sulfur content in organic acid esters of cellulose can be counteracted simply by soaking the ester in a very dilute aqueous solution containing magnesium or alkali metal ions. This treatment stabilizes the ester without effecting its viscosity. After soaking, the excess liquid is removed and the product is dried whereupon it exhibits the characteristics of a stabilized product, such as greater resistance to the action of heat, which indicates a longer life, than with cellulose esters which have not been stabilized.

It is an object of my invention to provide a method for stabilizing cellulose esters which is simple and inexpensive but is very efiective, which involves merely a soaking in a dilute aqueous solution of one of these salts. Various concentrations of these salts may be effective, however, I have found that it is only necessary to use very dilute solutions thereof in distilled water. For instance, a mass of cellulose acetate in fibrous condition was soaked in an excess of distilled 3 water containing .05% by weight of magnesium sulfate for one hour after which it was dried. The resistance of the resulting material to high temperatures was much greater than prior to this stabilization treatment.

40 Various water soluble salts of magnesium or the alkali metals such as sodium or potassium are suitable for use. For instance, instead of magnesium sulfate, the nitrate might be employed.

1 have found that the amount of stabilization 45 obtained in accordance with my invention depends to some extent onthe amount of sulfuric acid catalyst which was employed in making the cellulose esters. For instance, with various organic acid esters of cellulose in which only a 50 small amount of sulfuric acid catalyst was employed in their preparation, the charring points of individual esters were raised from 294 to 299 C., from 295 to 297 C. and from 275 to 278 C.,

respectively. However, when heated for 24 hrs.

at 180 F. the samples taken before the stabilization treatment charred while samples taken after the treatment remained unchanged.

In the case, however, of cellulose esters in the preparation of which a substantial proportion of sulfuric acid is employed so that the sulfur content present exerts a decided efiect upon the stability of the cellulose ester, the increase in resistance to heat of these esters, after treatment in accordance with the present invention, was considerable.

The test to determine the resistance to heat of a cellulose ester was as follows: The cellulose ester to be tested is placed in a test tube and maintained at a temperature of 180 C. for 8 hours. The sample is then removed from the tube and compared in color with samples of standard cellulose acetate which have been heated for different periods of time. The heat test figure gives the number hours of heating required by the 20 standard acetate to develop the color which the unknown acetate has after 8 hrs. heat treatment. For instance, if an ester which is heated for 8 hours has substantially the same coloration as a standard ester heated for 4 hours, the heat test value is 4. Thus it may be seen that a cellulose ester having a heat test figure below 8 is more resistant to heat than the standard cellulose acetate employed as a check. For instance, one ester which had been prepared in a process in which a substantial amount, such as more than 5%, based on the cellulose, of sulfuric acid catalyst had been used, showed an increase in charring point from 272 to 308. Another ester of the same type was stabilized by the present invention, as evidenced by an increase in the charring point from 271 to 291 and a lowering of the heat test figure from 8 to 4. In still another case of stabilization by the present method, the charring point of an organic acid ester of cellulose was increased from 268 to 296 and the heat test 0 figure was lowered from 6 to 4.

Not only is this stabilization method applicable to partially hydrolyzed cellulose acetate but also to other organic acid esters of cellulose, such as cellulose propionate, cellulose butyrate, cellulose acetate propionate and cellulose acetate butyrate, which have been partially hydrolyzed.

Proportions which may be satisfactorily employed in the washing of cellulose esters are one part of the ester soaked in about 20 parts by weight of distilled water containing a small quantity such as for instance .1% to .05% of dissolved salt. Usually a treatment for about 15 minutes at room temperature is all that is necessary for accomplishing the stabilization. At the end of 55 this time the solution may be drained from the ester and the ester centrifuged to remove any excess liquid. It is then preferably dried at a temperature of 140-220 F. If desired, the water may be either cold, at room temperature or warmed, but as there is no apparent advantage in varying from the use of normal temperature, that temperature is preferred in ordinary practice.

The esters stabilized by my invention may be employed for making photographic film, yarn, thin sheeting or the like, which will retain their life and flexibility over a long period of years.

I claim:

1. The method of stabilizing an unstable partially hydrolyzed lower fatty acid ester of cellulose which has been separated from the esteriflcation bath which comprises treating that ester with a dilute aqueous solution containing .1% to .05% of a salt having magnesium or alkali metal 10I1S.

2. The method of stabilizing an unstable partially hydrolyzed lower fatty acid ester of cellulose which has been separated from the esterification bath which comprises soaking the ester in a dilute aqueous solution having a concentration of .1% to .05% of a salt which supplies alkali metal ions thereto.

3. The method of stabilizing an unstable partially hydrolyzed lower fatty acid ester of cellulose which has been separated from the esterification bath which comprises soaking the ester in a dilute aqueous solution having a concentration of .1% to .05% of a salt supplying magnesium ions thereto.

CARL J. MALM. 

